The problem of iron carry-out is common in the case of electrostatic copying machines which employ a two-component developer composition comprising toner powder and a magnetizable carrier powder for said toner powder, such as an iron powder. The developer powder mixture is present in a developer station through which the latent-imaged photoconductive surface is moved. The developer station comprises a powder applicator roller which is charged with a negative voltage of 150 volts d.c., while the latent electrostatic images present on the photoconductive surface have a potential or negative 500 volts.
Due to the triboelectric effect of the powder particles rubbing together in the developer station, the iron powder particles become negatively charged and the toner powder particles become positively charged. The positive toner particles are attracted to the latent images, while the negative iron particles are not attracted, whereby only the toner particles transfer to the photoconductive surface for subsequent transfer to a copy sheet and fusion thereon as duplicate images.
In practice, various small amounts of the iron powder particles do, in fact, transfer to the photoconductive surface, causing abrasion damage to the photoconductive surface and/or degradation of the quality of the copies produced due to background staining and/or damage to the machine parts and/or waste of iron powder.
It is not clear why the iron powder transfers to the photoconductive surface. It appears that the separation of the toner particles from the iron particles leaves the iron particles with an excess of negative charges which causes the iron particles to be attracted to the electro-positive toner layer on the photoconductive surface. Regardless of theory, the problem exists and there have been proposals for various scavenger means for overcoming the problem.
U.S. Pat. Nos. 3,457,900 and 3,894,513 disclose devices for removing iron particles from a photoconductive surface comprising a magnet fixed within a rotating sleeve of nonmagnetizable material which must be positioned beneath the photoconductive surface in order to continuously release the attracted iron particles during the copying cycle.
U.S. Pat. No. 3,993,022 discloses a similar apparatus, including a complex conveyor system for removing iron particles from a master surface and conveying them to a collecting tray during the copying cycle.
U.S. Pat. No. 3,739,749 relates to another proposed apparatus for removing iron particles and causing them to move back to the developer station in response to the rotation of adjacent permanent magnets within a sleeve.
Such prior known devices, which add substantially to the cost and complexity of an electrostatic copying machine, are subject to breakdown and failure due to their many moving parts and due to the fact that such parts are in constant movement during the copying cycle.